Method for purifying lactames

ABSTRACT

The invention relates to a method for purifying lactames, especially lactames obtained by cyclization hydrolysis of an aminoalkylnitrile, and more particularly, to the purification of ε-caprolactame obtained by cyclization hydrolysis of the aminocapronitrile. This crystallisation process can either be carried out directly on the cyclization reaction medium after removal of the volatile components or on a medium that has been subjected to different treatments or on a caprolactame extracted from the reaction medium, for example by distillation.

[0001] The invention relates to a process for purifying lactams, inparticular the lactams obtained by cyclizing hydrolysis of anaminoalkylnitrile.

[0002] The invention relates more particularly to the purification ofε-caprolactam obtained by cyclizing hydrolysis of aminocapronitrile.

[0003] Lactams and in particular ε-caprolactam are important chemicalcompounds used as starting materials in the manufacture of manyproducts. One of the most important applications is the manufacture ofpolymers such as polyamides.

[0004] Among the processes for synthesizing lactams and in particularε-caprolactam which have been proposed is a process consisting inmanufacturing an aminoalkylnitrile such as aminocapronitrile and then incarrying out a cyclizing hydrolysis of this aminonitrile. This process,which may be carried out in liquid or gaseous medium with or withoutsolvent, is disclosed, for example, in patents EP 0 729 454, P 0 729453, EP 0 793 650, EP 0 793 651, EP 0 794 943, EP 0 815 077, EP 0 815078, EP 0 748 797, EP 0 6559 741 and WO 96/22974.

[0005] An important step in the manufacture of lactams, in particularε-caprolactam, when these need to be used as monomers in the manufactureof polymers such as polyamides, is the purification of the products.Specifically, for these applications, the lactam must satisfy highlystringent and demanding purity criteria. These criteria are defined byvarious indices or concentrations corresponding to several types orcategories of impurity. These main criteria are:

[0006] permanganate number (I_(KMnO) ₄ )

[0007] content of volatile bases (I_(VB))

[0008] UV index (I_(UV))

[0009] Various purification processes have already been proposed. Thus,U.S. Pat. No. 5 496 941 discloses a process for purifying caprolactamextracted from the reaction medium of a cyclizing hydrolysis bydistillation comprising a hydrogenation step followed by a treatment inacidic medium (treatment on ion-exchange resin or distillation insulphuric acid medium) and a distillation in basic medium.

[0010] European patent application No. 94/3608 discloses a process forpurifying crude ε-caprolactam obtained by cyclization of an alkyl6-aminocaproate, which consists in carrying out a crystallization of thecaprolactam. According to the said document, certain components such asN-methylcaprolactam, methylvalerolactam and valeramide are removed bycrystallization. The process disclosed indicates that the caprolactamthus crystallized satisfies the required purity criteria regarding thecontent of volatile bases and the UV index for absorption at awavelength of 290 nm.

[0011] However, the said document does not specify whether or not thethird purity criterion, the permanganate number, is achieved.

[0012] In the case of a caprolactam obtained from the cyclizinghydrolysis of an aminocapronitrile obtained by semihydrogenation ofadiponitrile, the purity criteria of the caprolactam are achieved bycarrying out purification processes described above comprising severalsuccessive treatments, in particular an acidic treatment and a finaldistillation in basic medium.

[0013] In such processes, the production of a caprolactam of the desiredpurity requires the implementation of a distillation operation with alarge heavy fraction (fraction of products with a high boiling point)comprising a large amount of lactam in the form of lactam or oligomers.This loss of lactam is detrimental to the process.

[0014] One of the aims of the present invention is especially toovercome this drawback by proposing a process for purifying lactamsmaking it possible to obtain a product that satisfies the desired puritycriteria, thus limiting the lactam treatment steps and more particularlythose which may generate the formation of oligomers.

[0015] To this end, the invention proposes a process for purifying alactam obtained from the cyclization of an aminoalkylnitrile obtained bysemihydrogenation of an alkyldinitrile, which consists, after removingthe volatile compounds from the cyclization medium, in extracting thelactam formed by crystallization.

[0016] According to another preferred characteristic of the invention,the crystalline lactam can be subjected to one or more othercrystallization steps to obtain the desired purity. Other purificationtreatments may be carried out on the crystalline lactam withoutdeparting from the scope of the present invention.

[0017] According to another embodiment, the cyclization medium may besubjected to a hydrogenation before the step to remove the volatilecompounds According to yet another embodiment, the crystallization ofthe caprolactam is carried out on the cyclization medium which may ormay not have undergone a hydrogenation, after treating this medium inacid medium, for example by passing it through an ion-exchange resin ora distillation in acidic medium.

[0018] According to another embodiment, a hydrogenation of thecyclization medium may be carried out after removing tie volatilecompounds.

[0019] For simplicity, the expression “cyclization medium” for carryingout the crystallization means either the reaction medium obtained fromthe cyclization step on which the various treatments described above arecarried out, or the medium formed by the caprolactam extracted after thecyclization reaction, for example by distillation or entrainment, inother words after removal or separation of the volatile compounds andthe high-boiling compounds. The classification into volatile compoundsand high-boiling compounds is made relative to the boiling point of thelactam to be recovered.

[0020] According to the invention, the extraction and recovery of thelactam by a crystallization make it possible to obtain a product whichsatisfies the desired purity criteria, without the need for treatment attemperatures which may generate the formation of oligomers, for example.

[0021] Thus, the degradation or polymerization of the lactam, whicharises when it is heated to high temperatures, especially in order todistil it, is avoided.

[0022] The process of the invention applies more particularly to thepurification of ε-caprolactam obtained by cyclizing hydrolysis ofaminocapronitrile.

[0023] This aminocapronitrile is synthesized by semihydrogenation ofadiponitrile. These various semihydrogenation processes are disclosed,for example, in patents EP 0 737 100, EP 0 737 181 and WO 96/18603.

[0024] According to the invention, the solvents that are suitable forthe crystallization may be a lactam, water, saturated hydrocarbons suchas hexane or cyclohexane, alcohols, aromatic hydrocarbons, organohalogencompounds such as CCl₄ or CHCl₃, ketones or the like.

[0025] Solvents that well preferably be used include water and/or alactam and more preferably a lactam which is identical to the one whichneeds to be purified, or, more preferably, a saturated aqueous lactamsolution.

[0026] The crystallization may be carried out in one step or in severalsteps. The mother liquor obtained in one step is advantageously recycledinto one of the preceding steps. More generally, the crystallization maybe carried out according to the conventional industrial crystallizationtechniques.

[0027] According to another characteristic of the process of theinvention, the compound obtained after crystallization is filtered off,spin-filtered or isolated by any other common solid/liquid separationtechnique. The solid recovered may be advantageously washed with asolvent, preferably with a saturated lactam solution.

[0028] Advantageously, the medium containing the lactam to becrystallized is concentrated by any means which is suitable to obtain aweight concentration of lactam of greater than 80% by weight, preferablygreater than 90%.

[0029] Thus, examples of techniques for saturating or concentrating thesolution which may be mentioned include techniques of cooling andevaporation advantageously under reduced pressure to work at lowertemperature.

[0030] In addition, the crystallization can be performed with seeding ofthe solution in order to promote nucleation of the lactam.

[0031] The pure lactam recovered after solid/liquid separation, forexample filtration, and optional washing, is dried or the solvent, forexample water, is distilled off according to usual techniques (when thesolvent is water, the caprolactam is dehydrated).

[0032] The process of the invention thus makes it possible to produce alactam, more particularly an ε-caprolactam, which has the desired puritycriteria for use as monomer in the manufacture of polyamides such asPA6. The polyamides thus obtained are suitable in particular for themanufacture of textile yarns or fibres with working properties that areequivalent to those of the current polyamides.

[0033] Other advantages and details of the invention will be illustratedby the implementation examples below, given purely as a guide.

EXAMPLE 1

[0034] An ε-caprolactam is manufactured according to the processdisclosed in the patents cited above.

[0035] Thus, this ε-caprolactam is obtained according to Example 1 ofpatent EP 0 748 797 by semihydrogenation of adiponitrile followed bycyclizing hydrolysis of aminocapronitrile according to the conditionsdescribed in the said example.

[0036] The medium obtained is subjected to a hydrogenation according tothe process disclosed in the as yet unpublished trench patentapplication filed under Ser. No. 98/14735 on Nov. 19, 1998.

[0037] The ammonia present in the hydrogenated cyclizing hydrolysisreaction medium is then removed by evaporation or entrainment.

[0038] The cyclization medium thus obtained is then treated by passingit through an ion-exchange resin according to the conditions disclosedin the patents or patent applications WO 98/05636 or WO 96/20923.

[0039] The medium obtained after treatment with resin contains 65% bvweight of ε-caprolactam and numerous other products which were not allidentified.

[0040] The medium thus recovered is analysed to determine the puritycriteria defined above. The results of these analyses are indicated inthe table below.

[0041] In accordance with the process of the invention, the caprolactamis recovered by crystallization from this solution after concentrationunder reduced pressure at a caprolactam content in the region of 90% byweight.

[0042] The solution is concentrated at a temperature of 40° C. and isthen cooled to a temperature in the region of 20° C. at a cooling rateof about 10° C. per hour.

[0043] The crystals formed are recovered by filtration on a sinterfunnel. The mother liquors recovered are stored for possible recycling.

[0044] The sold cake of caprolactam crystals is washed with a saturatedaqueous caprolactam solution. The degree of washing with wet base isadvantageously between 1 and 5.

[0045] The crystalline caprolactam is analysed. The puritycharacteristics are collated in Table I below. Test Medium I_(UV)I_(KMO4) I_(VB) (meq/kg) Medium before 0.45 32.06 2.69 crystallizationWet cake after 0.02 2.03 0.04 washing

EXAMPLE 2

[0046] A caprolactam is manufactured according to the process describedin Example 1 and with reference to the process disclosed in patentapplication EP 0 748 797.

[0047] However, in this example, the medium obtained from the cyclizinghydrolysis is only treated to remove the volatile compounds, moreparticularly ammonia.

[0048] The crystallization of the caprolactam formed is carried outdirectly using this solution after concentration by evaporation ofwater.

[0049] The crystallization is carried out in two successive steps. Thecake recovered in the first step is taken un to give a new saturatedaqueous solution and is then recrystallized. The purity characteristicsof the cake recrystallized after washing are given in Table II below.Test Medium I_(UV) I_(KMO4) I_(VB) (meq/kg) 2 Medium before 17.7 400 130crystallization Wet cake 0.03 2.0 0.20 recrystallized after washing

1. Process for purifying lactams obtained by cyclization of anaminoalkylnitrile, characterized in that it consists in recovering thelactam by crystallization after removing the volatile compounds from thecyclization medium.
 2. Process according to claim 1, characterized inthat the crystallization is carried out in several steps.
 3. Processaccording to claim 1 or 2, characterized in that the cyclization mediumis subjected to a hydrogenation before removing the volatile compounds.4. Process according to one of the preceding claims, characterized inthat the cyclization medium is treated with an ion-exchange resin beforerecovering the lactam by crystallization.
 5. Process according to one ofclaims 1 to 3, characterized in that the cyclization medium is treatedby distillation in sulphuric medium.
 6. Process according to one of thepreceding claims, characterized in that the cyclization medium fromwhich the lactam is crystallized consists of the reaction mediumobtained from the cyclization reaction after removal of the volatilecompounds and the high-boiling compounds.
 7. Process according to one ofclaims 1, 2 and 4 to 6, characterized in that the cyclization medium issubjected to a hydrogenation after separation of the volatile compoundsand/or of the high-boiling compounds.
 8. Process according to one of thepreceding claims, characterized in that the lactam is ε-caprolactam. 9.Process according to claim 8, characterized in that the ε-caprolactam isproduced by cyclizing hydrolysis of aminocapronitrile.
 10. Processaccording to claim 9, characterized in that the aminocapronitrile issynthesized by semihydrogenation of adiponitrile.
 11. Process accordingto one of the preceding claims, characterized in that thecrystallization solvent is chosen from the group comprising a lactam, anaqueous lactam solution, water, saturated hydrocarbons, alcohols,aromatic hydrocarbons, organohalogen compounds and ketones
 12. Processaccording to claim 11, characterized in that the solvent is a saturatedaqueous lactam solution.
 13. Process according to one of the precedingclaims, characterized in that the weight concentration of lactam in thesolution to be crystallized is greater than 80% by weight.